The present exemplary embodiments relate to printing systems with raster output scanner (ROS) apparatus and to techniques for mitigating banding errors. Reprographic printing systems are used to create marked images on paper or other remarkable media, and improving the quality of the produced images is a continuing goal. Final image quality is affected by various sources of noise and errors in a reprographic system, leading to density variations in the marking material fused to the final print medium. In the reprographic process, a photoreceptor travels along a process direction, and images and text are formed as individual scan lines or groups of scan lines (sometimes referred to as a swath) in a raster scanning process in a cross-process direction, where the process direction motion is much slower than the raster scanning in the cross-process direction. Accordingly, the cross-process scanning direction is sometimes referred to as a “fast scan” direction, and the process direction is referred to as a “slow scan” direction.
Certain sources of reprographic system noise and errors caused periodic density variations in the process direction, which are sometimes referred to as “banding” errors. Periodic density variations may be characterized by frequency, amplitude, and phase in relation to a fundamental frequency, as well as harmonics. Various sources of banding exist in a marking (or print) engine. For example, raster output scanners employ rotating polygon mirror apparatus driven by a motor, known as a motor polygon assembly or MPA, with one or more light sources being scanned by rotation of the MPA such that scan lines are generated in the fast scan (cross-process) direction through reflection off a reflective facet of the rotating polygon mirror apparatus.
Differences in reflectivity, shape, profile, orientation, etc. in different reflective facets of the polygon lead to differences in image density (color intensity) in the final print out which are a function of which polygon facet was used to create a given scan line or swath of scan lines. As a result, the final print image may include bands of variations from the desired density that are periodic in the process direction. Other sources of banding errors include gears, pinions, and rollers in charging and development modules; jitter and wobble in imaging modules, as well as photoreceptors and associated drive trains. Banding usually manifests itself as periodic density variations in halftones in the process direction. The period of these defects is related to the once around frequency of the banding source. If not addressed, such periodic process direction density variations can render a reprographic printing system unacceptable, particularly where the banding errors are visually perceptible.
Banding can be addressed through reductions in the sources of such noise or errors and/or by compensation in various reprographic system components in order to counteract its affects, typically by injecting a known error that offsets the banding resulting from the sources of such periodic density variations. There are many various errors that produce banding at the 1× (and multiples) of the revolution frequency of the MPA (motor polygon assembly) in reprographic systems using a raster output scanner. In practice, it is difficult to completely eliminate the error sources that contribute to MPA harmonic banding, or even to reduce them enough to avoid perceptible periodic density variations. In addition, customer requirements are continually reducing the amount banding that is deemed to be acceptable. Consequently, banding compensation techniques have become an important tool in meeting reprographic system performance specifications. For instance, ROS exposure power can be varied in a controlled fashion to compensate for banding, and conventional banding compensation techniques include measurement of banding (including from multiple sources) and the use of that information to actuate some correction strategy on a scanline by scanline basis (including ROS exposure variation) to combat banding. However, conventional banding compensation approaches do not address cross-process (fast scan) direction density variation in banding, and instead average test prints in the cross-process direction to get a one-dimensional banding profile which is then used to derive the banding compensation independent of cross-process banding density variation information).
The following documents are incorporated by reference in their entireties: U.S. Pat. App. Publication No. 2011/0058186 to Ramesh et al., filed Sep. 8, 2009, Least Squares Based Coherent Multipage Analysis of Printer Banding for Diagnostics and Compensation; U.S. Pat. App. Publication No. 2011/0058226 to Ramesh et al., filed Sep. 8, 2009, Banding Profile Estimation using Spline Interpolation; U.S. Pat. App. Publication No. 2011/0058184 to Ramesh et al., filed Sep. 8, 2009, Least Squares Based Exposure Modulation for Banding Compensation; U.S. Pat. App. Publication No. 2007/0052991 to Goodman et al., filed Sep. 8, 2005, Methods and Systems for Determining Banding Compensation Parameters in Printing Systems; U.S. Pat. App. Publication No. 2009/0002724 to Paul et al., filed Jun. 27, 2007, Banding Profile Estimator using Multiple Sampling Intervals; U.S. Pat. App. Publication No. 2007/0139509 to Mizes et al., filed Dec. 21, 2005, Compensation of MPA Polygon Once Around with Exposure Modulation; U.S. Pat. App. Publication No. 2007/0236747 to Paul et al., filed Apr. 6, 2006, Systems and Methods to Measure Banding Print Defects; U.S. Pat. No. 7,120,369 to Hamby et al.; U.S. Pat. No. 7,058,325 to Hamby et al; U.S. Pat. No. 5,519,514 to TeWinkle; U.S. Pat. No. 5,550,653 to TeWinkle et al.; U.S. Pat. No. 5,680,541 to Kurosu et al.; U.S. Pat. No. 6,621,576 to Tandon et al.; U.S. Pat. No. 6,432,963 to Yoshino; U.S. Pat. No. 6,462,821 to Borton et al.; U.S. Pat. No. 6,567,170 to Tandon et al., U.S. Pat. No. 6,975,949 to Mestha et al.; U.S. Pat. No. 7,024,152 to Lofthus et al.; U.S. Pat. No. 7,136,616 to Mandel et al.; U.S. Pat. No. 7,177,585 to Matsuzaka et al.; and U.S. Pat. No. 7,492,381 to Mizes et al.